This disclosure concerns an invention relating generally to plasma generators, and more specifically to xe2x80x9ccoldxe2x80x9d plasma generators and/or plasma generators operating at atmospheric pressure.
Plasma, the fourth state of matter, consists of gaseous complexes in which all or a portion of the atoms or molecules are dissociated into free electrons, ions, free radicals, and neutral particles. On earth, plasma occurs naturally in lightning bolts, flames, and similar phenomena, or may be manufactured by heating a gas to high temperatures, or by applying a strong electric field to a gas, the more common method. The latter type of plasma, often referred to as an electrical discharge plasma, can be further sub-classified as a xe2x80x9chotxe2x80x9d plasma, i.e., dissociated gas in thermal equilibrium at high temperatures (xcx9c5000K), or xe2x80x9ccoldxe2x80x9d plasma, i.e., nonthermal plasma wherein the dissociated gas is at low temperatures but its electrons are at high temperature (i.e., in a state of high kinetic energy).
The usefulness of plasma for manufacturing and other applications is best understood by reviewing common applications for cold plasma. As an example, common cold plasma processing methods are commonly used to alter the surface properties of industrial materials without affecting the bulk properties of the treated material. The most common cold plasma surface treatments may be generally categorized as cleaning, activation, grafting, and deposition processes, each of which will now be briefly reviewed.
Plasma cleaning processes typically utilize inert or oxygen plasmas (i.e., plasmas generated from inert or oxygen-based process gases) to remove contaminants (generally organic contaminants) on a material surface subjected to vacuum. The contaminants are exposed to a plasma stream, and they undergo repetitive chain scission from the plasma until their molecular weight is sufficiently low to boil away in the vacuum.
Plasma activation is used when a material (generally a polymer or elastomer) is subjected to a plasma generally produced from an inert or non-carbon gas, and results in the incorporation of different moieties of the process gas onto the surface of the material being treated. For example, the surface of polyethylene normally consists solely of carbon and hydrogen. However, if subjected to an appropriate plasma, the surface may be activated to contain a variety of functional groups which enhance the adhesion and permanence of coatings later applied to the surface. As an example, a surface can be treated to greatly enhance its ability to bond with adhesives.
Deposition, which is exemplified by a process referred to as plasma-enhanced chemical vapor deposition (PECVD), utilizes a complex molecule as the process gas. The process gas molecules are decomposed near the surface to be treated, and recombine to form a material which precipitates onto and coats the surface.
Grafting generally utilizes an inert process gas to create free radicals on the material surface, and subsequent exposure of the radicalized surface to monomers or other molecules will graft these molecules to the surface.
The foregoing cold plasma processes have numerous practical applications, including sterilizing of medical equipment, application of industrial and commercial coatings, etching computer chips, semiconductors, and circuits, and so forth. Hot plasma might be used for generally the same types of applications as cold plasma. However, hot plasma applications are limited since most organic matter cannot be treated under the high temperatures required for hot plasmas without severe degradation. Additionally, hot plasma technology is energy and equipment intensive, making it expensive and difficult to work with. In contrast, cold plasma may be used at temperature ranges as low as room temperature (or lower), making it significantly easier to handle. However, cold plasma processes have the disadvantage that they generally need low pressure conditions to operate (generally a vacuum), and consequently need large, static (i.e., immobile) equipment with a low-pressure treatment chamber to operate. This causes significant manufacturing constraints since the need to treat items within an enclosed chamber makes it inherently difficult to process the items continuously in assembly-line fashion, as opposed to processing the items in batches.
Some of these difficulties have been overcome with further developments in dielectric barrier discharge (DBD) plasma production processes. These processes, which may take place at room temperature and non-vacuum conditions, space a pair of electrodes apart across a free space, with one or more dielectric layers also being situated between the electrode. When an alternating high voltage electrical current is applied to the plates, xe2x80x9cmicroburstsxe2x80x9d of plasma are generated from the gas(es) in the free space. DBD apparata are sometimes used to generate ozone by ionizing oxygen passing through the free space of the apparatus, or to break apart volatile gaseous organic compounds passing through the free space. However, conventional DBD plasma generation apparata are not well suited for surface treatment of workpieces because of the difficulty in transporting the workpieces through the free space without the plasma""s interference with the transport mechanism; for example, one generally cannot run a conveyor through the free space. Plasma processes using DBD are further limited by the size constraints that the free space imposes on the workpieces. Since the free space is relatively small, the size range of workpieces that can be treated is correspondingly small, which greatly limits usage.
Thus, it would be useful to have available methods and apparata of generating cold plasma at low pressures (including at and/or around atmospheric pressure) while alleviating or eliminating the disadvantages of prior cold plasma equipment and methods.
The invention involves a plasma generator which is intended to at least partially solve the aforementioned problems. To give the reader a basic understanding of some of the advantageous features of the invention, following is a brief summary of preferred versions of the plasma generator. As this is merely a summary, it should be understood that more details regarding the preferred versions may be found in the Detailed Description set forth elsewhere in this document. The claims set forth at the end of this document then define the various versions of the invention in which exclusive rights are secured.
The plasma generator includes several plasma sources distributed in an array for plasma treatment of surfaces. Each plasma source includes spaced first and second conductive electrodes between which plasma will be generated. Each second electrode has a gas passage defined therein, and one of the first electrodes is situated within the gas passage in spaced relation from the second electrode, with the gas passage thereby constituting the free space for plasma generation. As an example, each second electrode may be formed as a hollow cylinder having an interior gas passage, and each first electrode may be formed as a rod which is concentrically situated within a second electrode""s gas passage spaced from the gas passage walls. An insulating layer is interposed between the first and second electrodes, as by providing a ceramic coating on the surfaces of the first electrodes and/or upon the gas passage walls of the second electrodes, to facilitate plasma formation via dielectric barrier discharge (DBD) in the gas passages between the first and second electrodes.
The first electrodes may be provided on a common bed so that they protrude therefrom, with their bases affixed to the common bed and their tips being spaced from the common bed. This monolithic or integrally affixed first electrode structure, wherein the common bed may take the form of a plate having the first electrodes extending therefrom as groups of adjacently-spaced protrusions, therefore effectively connects the first electrodes together in a parallel electrical relationship. Similarly, the second electrodes may be formed by defining the gas passages within a common second electrode member, with the gas passages extending from a common inlet surface on the second electrode member to a common outlet surface on the common second electrode member. For example, the second electrode member may be formed as a plate having a series of gas passages formed as holes extending through the plate.
The first electrode bed may then be situated adjacent the inlet surface of the second electrode member, with its first electrodes situated within the gas passages so that the tips of the first electrodes extend towards the outlet surface of the second electrode member. The space between the bed of the first electrodes and the inlet surface of the second electrode member defines a gas plenum space onto which the gas passages open. Thus, supplying process gas(es) to the plenum space will in turn provide the process gas to the gas passages to travel between the first and second electrodes for plasma generation.
Apart from providing an insulating layer between the first and second electrodes, an insulating layer may also be provided on or adjacent to the inlet surface of the second electrode member, and/or the common bed of the first electrodes, to prevent arcing between the second electrode member and the first electrode bed. Insulation of the first and second electrodes is preferably done by casting ceramic material on or about the portions of the electrodes and/or surrounding structure which are desirably insulated. If desired, silicone or other expandable/compressible coatings may be applied prior to application of the ceramic material so that if the underlying structure undergoes thermal expansion or contraction, the expandable/compressible coating will prevent transmission of stresses to the ceramic insulation.
A gas supply may then be used to communicate process gas to the gas passages of the second electrodes, as by connecting a gas supply to the plenum space. Plasma generated in process gas situated in the gas passages between the first and second electrodes is ejected from the gas passages onto a workpiece situated adjacent the outlet surfaces of the second electrodes. A gas distributor for equalizing or otherwise tailoring the gas supply to the gas passages can be situated between the gas supply and the gas passages so that the desired amount of process gas will be supplied to each plasma source (i.e., to each pair of first and second electrodes). This can be done, for example, by situating a porous barrier in the plenum space between the gas supply and the gas passages so that each gas passage has approximately the same inlet pressure, and thus receives approximately the same amount of process gas.
The foregoing arrangement allows a workpiece to be situated adjacent to the outlet surfaces of the second electrodes so that plasma generated in the gas passages will impinge on the workpiece surface. Since the workpiece does not travel through the free space wherein the plasma is generated (i.e., the gas passages), the size of the workpiece is not limited by the size of the free space. Additionally, conveyors or other positioning means for adjusting the location of a workpiece with respect to the plasma sources may be accommodated since the workpiece and its positioning means need not be situated in the gas passages. The positioning means may allow transport of successive workpieces past the plasma sources for plasma treatment in assembly-line fashion, and/or may allow the plasma generator to be repeatedly translated over the same or different areas of a workpiece surface for more complete treatment. Unlike some prior plasma generators, the present plasma generator may be constructed in a sufficiently lightweight and compact unit that it can be readily moved over workpieces or from area to area, and can even be made in a handheld unit.
The plasma sources are preferably situated on the plasma generator in such a manner that when a workpiece is translated with respect to the plasma generator, each unit area of the workpiece travels adjacent to at least one plasma source for treatment. Stated differently, it is preferred that the plasma sources be arrayed in such a manner that xe2x80x9clanesxe2x80x9d of untreated workpiece surface do not result when the surface travels adjacent to the plasma generator.
Where the outlet surfaces of the second electrodes are arrayed along a surface (e.g., along the outlet surface of the second electrode member), they are preferably surrounded by a ledge so that when the ledge is situated adjacent a workpiece to be treated, the ledge defines an at least substantially enclosed chamber between the outlet surfaces and the workpiece, with the chamber being bounded by the outlet surface, the ledge, and the workpiece. This enclosed chamber contains the exhaust produced by the plasma treatment process, and exhaust outlets situated on or about the ledge can be provided to remove the exhaust from the chamber at the same time plasma is injected therein. Since exhaust to the surroundings may be eliminated or greatly reduced, this allows use of the plasma generator without significant ventilation equipment, e.g., one need not enclose and vent an entire conveyor line of workpieces that are being subjected to plasma treatment.
Advantageously, the structure of the plasma generator also allows it to be readily adapted to accommodate heating and/or cooling components, so that the process gas being used for plasma generation can be heated or cooled prior to or during plasma generation to obtain desired effects. As an example, enclosed fluid passages for carrying heat-exchanging fluids may be formed within one or more of the second electrode member (and/or its electrodes), the common bed (or its first electrodes), any insulating layers formed on or between the foregoing components, and/or on any framework associated with the foregoing components. The ability to provide heating and/or cooling components directly within the electrodes and/or their associated structure saves space and better provides for the ability to densely array the plasma sources, leading to more complete surface treatment of workpieces.
Test results have demonstrated that plasma generators using at least some of the foregoing features allow highly efficient plasma treatment of even very large workpiece surfaces, including surfaces of conveyorized workpieces. Additionally, the plasma generator is believed to provide a substantially uniform plasma emission over a greater surface area than known prior plasma generators, possibly owing to the density at which the plasma sources may be situated, and/or the high efficiency of the concentric electrode arrangement used in the preferred versions of the invention described in this document. Further advantages, features, and objects of the invention will be apparent from the following detailed description of the invention in conjunction with the associated drawings.